Abstract

Low‐frequency vocalizations were recorded from fin whales, Balaenopteraphysalus, in the Gulf of California, Mexico, during three cruises. In March 1985, recorded 20‐Hz pulses were in sequences of regular 9‐s interpulse intervals. In August 1987, nearly all were in sequences of doublets with alternating 5‐ and 18‐s interpulse intervals. No 20‐Hz pulse sequences of any kind were detected in February 1987. The typical pulse modulated from 42 to 20 Hz and its median duration was 0.7 s (1985 data). Most other fin whale sounds were also short tonal pulses averaging 82, 56, and 68 Hz, respectively, for the three cruises; 89% were modulated in frequency, mostly downward. Compared to Atlantic and Pacific Ocean regions, Gulf of California 20‐Hz pulses were unique in terms of frequency modulation, interpulse sound levels, and temporal patterns. Fin whales in the Gulf may represent a regional stock revealed by their soundcharacteristics, a phenomenon previously shown for humpback whales, birds, and fish. Regional differences in fin whale sounds were found in comparisons of Atlantic and Pacific locations.